Optical transmission systems, devices, and methods

ABSTRACT

An optical system with a first and second network tiers. The first network tier includes a plurality of major nodes optically interconnected by at least one transmission path. The second network tier includes a plurality of minor nodes disposed along the transmission path and the minor nodes are connected to at least one of the major nodes. The minor node is configured to transmit all traffic to an adjacent major node, and the major nodes are configured to transmit to and receive information from other major nodes and minor nodes on transmission paths connected to the major node.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional PatentApplication No. 60/300,746 filed on Jun. 25, 2001.

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention is directed generally to opticaltransmission systems. More particularly, the invention relates towavelength allocation in multidimensional wavelength divisionmultiplexed (“WDM”) optical transmission systems.

[0004] Digital technology has provided electronic access to vast amountsof information. The increased access has driven demand for faster andhigher capacity electronic information processing equipment (computers)and transmission networks and systems to link the processing equipment.

[0005] In response to this demand, communications service providers haveturned to optical communication systems, which have the capability toprovide substantially larger information transmission capacities thantraditional electrical communication systems. Information can betransported through optical systems in audio, video, data, or othersignal formats analogous to electrical systems. Likewise, opticalsystems can be used in telephone, cable television, LAN, WAN, and MANsystems, as well as other communication systems.

[0006] Early optical transmission systems, known as space divisionmultiplex (SDM) systems, transmitted one information signal using asingle wavelength in separate waveguides, i.e. fiber optic strand. Thetransmission capacity of optical systems was increased by time divisionmultiplexing (TDM) multiple low bit rate, information signals into ahigher bit rate signal that can be transported on a single opticalwavelength. The low bit rate information carried by the TDM opticalsignal can then be separated from the higher bit rate signal followingtransmission through the optical system.

[0007] The continued growth in traditional communications systems andthe emergence of the Internet as a means for accessing data has furtheraccelerated the demand for higher capacity communications networks.Telecommunications service providers, in particular, have looked towavelength division multiplexing (WDM) to further increase the capacityof their existing systems.

[0008] In WDM transmission systems, pluralities of distinct TDM or SDMinformation signals are carried using electromagnetic waves havingdifferent wavelengths in the optical spectrum, i.e., far-UV tofar-infrared. The pluralities of information carrying wavelengths arecombined into a multiple wavelength WDM optical signal that istransmitted in a single waveguide. In this manner, WDM systems canincrease the transmission capacity of existing SDM/TDM systems by afactor equal to the number of wavelengths used in the WDM system.

[0009] Optical WDM systems are presently deployed as in point-to-pointWDM serial optical links (“PTP-WDM”) interconnected by electricalregenerators and switches. At each regenerator in the PTP-WDM systems,the information being transmitted can be merely regenerated on the samewavelength and retransmitted through the next link or electricallyswitched to one of a plurality of links, different fiber, and/or adifferent wavelength. Various electrical switch devices can be used toswitch the information between the different links at each regenerationsite.

[0010] As would be expected, the cost of performingoptical-electrical-optical conversions in PTP-WDM systems becomesextremely expensive merely to route traffic through a network. The costof electrical regeneration/switching in WDM systems will only continueto grow with WDM systems having increasing number of optical signalchannels, or wavelengths. As such, there is a desire to eliminateunnecessary, and costly, electrical switching of information beingtransported in optical systems.

[0011] Numerous optical cross-connect switches have been proposed asalternatives to electrical switching. For example, U.S. Pat. Nos.4,821,255, 5,446,809, 5,627,925 disclose various optical switch devices.A difficulty with optical cross-connect switches is that the switchesbecome overly complex as the number of optical channels and input/outputports on the device is increased.

[0012] As the need for high capacity WDM systems continues to grow, itwill become increasingly beneficial to provide all-optical networks thateliminate the need for electrical conversion to perform signal routingand grooming in the networks. The development of multi-dimensionalall-optical networks will provide the cost and performancecharacteristics required to further development of high capacity, moreversatile, longer distance communication systems.

BRIEF SUMMARY OF THE INVENTION

[0013] The systems, devices, and methods of the present inventionaddress the above-stated needs. In one embodiment, the present inventionis an optical system including a first network tier including aplurality of major nodes optically interconnected by at least onetransmission path, and a second network tier including a plurality ofminor nodes disposed along the transmission path and the minor nodes areconnected to at least one of the major nodes, wherein the minor node isconfigured to transmit all traffic to an adjacent major node and themajor nodes are configured to transmit to and receive information fromother major nodes and minor nodes on transmission paths connected to themajor node.

[0014] In another embodiment the present invention is an optical systemincluding a plurality of major nodes optically interconnected by atleast one transmission path, and at least one minor node disposed alongthe transmission path connected to at least one of the major nodes,wherein the minor node is configured to transmit all traffic to anadjacent major node and the major nodes are configured to transmit toand receive information from other major nodes and minor nodes ontransmission paths connected to the major node.

[0015] In another embodiment, the present invention is a method foroptically communicating in an optical network having first and secondnetwork tiers including a plurality of major and minor nodes,respectively, interconnected by at least one optical communicationspath, including receiving at a major node traffic from a minor node,aggregating traffic received at the major node from the minor node, andtransmitting traffic from the major node to another node in the network.

[0016] Those and other embodiments of the present invention will bedescribed in the following detailed description. The present inventionaddresses the needs described above in the description of the backgroundof the invention by providing improved systems, apparatuses, andmethods. These advantages and others will become apparent from thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings for thepurpose of illustrating embodiments only and not for purposes oflimiting the same, wherein:

[0018]FIG. 1 shows a system or network of the present invention;

[0019] FIGS. 2-5 depict optical communication systems of the presentinvention;

[0020] FIGS. 6-9 b depicts waveband selectors of the present invention;

[0021] FIGS. 10-12 depict transient grating waveband selectors of thepresent invention;

[0022] FIGS. 13-14 depict multi-node optical communication networks ofthe present invention;

[0023]FIG. 15 shows an overview block diagram of methods of the presentinvention;

[0024] FIGS. 16-18 show routing methods according to the presentinvention;

[0025]FIG. 19 shows an example of a method for selecting a regenerationsite;

[0026] FIGS. 20-23 show methods of forming spectral groups;

[0027]FIG. 24 shows a method of assigning spectral group numbers;

[0028]FIGS. 25a-25 d show methods of combining spectral groups;

[0029]FIG. 26 shows a network according to an embodiment of the presentinvention; and

[0030]FIG. 27 shows protection and working paths for minor nodesaccording to the present invention.

DESCRIPTION OF THE INVENTION

[0031]FIG. 1 illustrates an embodiment of an optical network or system10 including a plurality of optical nodes 14 connected by opticalcommunication paths 26. The system 10 can employ one or moretransmission schemes, such as space division multiplexing, time divisionmultiplexing, wavelength division multiplexing, etc. The system 10 willbe described as being “all-optical”, although advantages of the presentinvention can be realized with other than all-optical systems.

[0032] All-optical systems are those in which optical communicationpaths 26 are uninterrupted throughout the system 10 so that signals cantravel between nodes 11 and through nodes 11 without undergoing anoptical-electrical-optical conversion. When signals in all-opticalsystems are to be regenerated, they are removed from the opticalcommunication paths 26 of the system 10 without terminating the opticalcommunication path 26, so that other signals that do not need to beregenerated can continue without having to undergoing anoptical-electrical-optical conversion. Optical signals to be regeneratedare removed from the optical communication path 26, converted toelectrical form, regenerated, converted back into optical form, and theregenerated optical signal is inserted on the optical communication path26 of the system 10.

[0033] The optical nodes 11 can include various optical signalprocessing devices, such as transmitters and receivers (which can becollocated to form an optical network gateway (“ONG”)), opticalcross-connect switches or routers (“OXC”), amplifiers, and opticaladd/drop multiplexers (“OADMs”). ONGs are used to terminate an opticalcommunication path 26, as well as interface between two systems 10,convert signals from the optical domain to the electrical domain, andconvert signals from the electrical domain to the optical domain,whereas OXCs and OADMs allow signals to be selectively added and/ordropped from a communication path 26 without terminating the path 26.The system 10 can also include signal processing devices in locationsother than the nodes 11, such as amplifiers located between the nodes11.

[0034] The optical nodes 11 are generally configured to switch, route,demultiplex, multiplex, convert, and/or terminate pluralities of opticalsignal channels as groups, referred to as wavebands or “spectralgroups”. Examples of signal processing devices which process signalchannels as spectral groups are described in more detail hereinbelowwith respect to FIGS. 2-14, and such devices will be referred to asspectral group routers (“SGRs”). Each SGR will typically have thecapacity for a plurality of spectral groups. For example, an SGR 11 maybe designed to process up to one hundred or more signal channels usingfive, ten, twenty-five, fifty or more spectral groups, each includingone or more signal channels within each spectral group.

[0035] A spectral group is one or more signal channels that share acommon path or portion of the system 10, although all signal channels ina spectral group do not necessarily have to share the identical path. Asa result of commonality within a spectral group, efficiencies can berealized with a system 10 in which SGRs 11 process signal channels asspectral groups (e.g., all signal channels in a spectral group areswitched to the same output path, or all signal channels in a spectralgroup are added or dropped from the system 10), rather than asindividual channels. As a result, the SGR 11 processing signal channelsfrom an optical communication path 26 has only as many processingoperations to perform as there are spectral groups, regardless of thenumber of signal channels being carried by that path 26.

[0036] The optical communication paths 26 can include various guided andunguided media to provide for communication between the network nodes 11in the system 10. The optical communication paths 26 generally includeone or more optical fibers forming the paths between network nodes 11.Each path 26 can carry one or more uni- or bi-directionally propagatingoptical signal channels, or wavelengths. An optical link is thecommunication path 26 between two nodes 11.

[0037] In all-optical systems 10, signal channels or wavelengths must beallocated to each node 11, such that the signal channels used by somenodes 11 do not conflict with signal channels used by other nodes 11.When SGRs are used, the spectral groups and the signal channelscontained therein, must be allocated in a manner such that they do notcontend with other spectral groups and other signal channels. Aparticular spectral group may extend through an entire system 10, or itmay extend through only a portion of a system 10. Because the number ofspectral groups that can be handled by the nodes 11 is limited, it isoften advantageous to organize the system 10 so as to maximize thenumber of channels in each spectral group. To that end, the system 10can be organized into several sub-networks to allow convenient groupingof channels into spectral groups.

[0038] The sub-network for one spectral group may be different than asub-network for another spectral group passing through the same portionof the system 10. However, the edges of one spectral group mustinterface with another spectral group, or the signals carried on thatspectral group will not continue to propagate through the system 10. Atthe interface of these spectral groups, the signal channels whichcontinue to propagate will enter a new spectral group. Suchtransitioning of channels between spectral groups is accomplished by theSGRs. Information can be carried on one signal channel in spectral groupand then carried by another signal channel in the next spectral group,although such transitions are not required. Changing the channel onwhich the information is carried can be effected by dropping the signalchannel at a node 11, performing an optical to electrical conversion,and then performing an electrical to optical conversion onto a differentoptical signal channel. Alternatively, signal channels can be changedoptically by performing an optical wavelength conversion. Otherprocessing, such as filtering, reshaping, and retiming of the signal mayalso be performed.

[0039] A spectral group can be defined in terms of the sub-network inwhich it exists. A spectral group is terminated at one or more nodes 11defining the edge of, and bounding, the sub-network. Because thespectral groups are terminated at the edges of the sub-network, spectralgroups can be reused in other sub-networks in the system 10.

[0040] Spectral groups allow the size, cost, and complexity of networkelements 11 (e.g., SGRs) to be reduced. Of course, a system 10 utilizingsuch network elements will typically contain many spectral groups, andthe manner in which those spectral groups are formed and ordered isimportant to the efficient and proper operation of the system 10. Thoseand other features must be considered when forming systems 10 and SGRsusing spectral groups. Methods of forming such systems 10 and SGRs aredescribed hereinbelow with respect to FIGS. 15-29.

[0041]FIG. 2 shows a more detailed portion of the system 10. Generally,the optical system 10 has nodes 11 in the form of at least one opticaltransmitter 12 and at least one optical receiver 14, as shown in FIG. 2.Each transmitter 12 is configured to transmit information via one ormore information carrying wavelengths 18 _(l,k) contained in at leastone waveband 16 _(l,i) to the receivers 14. Each receiver 14 isconfigured to receive the information carried via one or more of theinformation carrying wavelengths 18 _(i,k). As used herein, the term“information” should be broadly construed to include any type of data,instructions, or signals that can be optically transmitted.

[0042] As shown in FIG. 2, the system 10 further includes at least oneintermediate optical processing node 20, such as an optical switch 22.The transmitter 12 is configured to transmit an optical signal 24containing one or more information carrying wavelengths 18 _(j) alongsignal transmission waveguide, i.e., fiber, 26 to the switch 22 viainput port 28. The optical processing node 20 includes one or morewaveband selectors, or selective element, 30 that are configured to passand/or substantially prevent the passage of information in wavebands 16_(i) to the receiver 14 via output ports 32. Because the information isbeing manipulated in wavebands, the individual information carryingwavelengths 18 _(j) within the waveband 16, do not have to be separatedin individual wavelengths to be managed and processed. Also, theindividual wavelengths 18 _(j) within the waveband 16 _(i) be varied inthe system 10 without affecting the configuration of the opticalprocessing node 20. Wavelengths 18 _(j) in the original signal 24 butnot within the waveband 16 _(i) are prevented from passing through tothe receivers 14.

[0043] In the present invention, optical signals 24 can be producedincluding a number of wavebands 16, each of which may contain one ormore information carrying wavelengths in a continuous band ofwavelengths or a plurality of wavelength bands. For example, a waveband16 can be defined as having a continuous range of ˜200 GHz containing 20different information carrying wavelengths 18 ₁₋₂₀ spaced apart on a 10GHz grid. The bandwidth of each waveband can be uniformly or variablysized depending upon the network capacity requirements. Likewise, thebandwidth of the waveband is not restricted, but can be varied toaccommodate varying numbers of wavelengths.

[0044] Generally, systems 10 of the present invention are configured sothat the optical processing nodes do not separate and process individualinformation carrying wavelengths during transmission from thetransmitter to the receiver. Instead, optical processing nodes 20 areconfigured to process the information in wavebands that may include anynumber of individual information carrying wavelengths. The processing ofinformation in wavebands decreases the complexity involved in processinglarge numbers of channels, while increasing the flexibility of opticalcomponents deployed in the transmission path between transmitters andreceivers. The bandwidth and number of information carrying wavelengthswithin a waveband in a network can be statically or dynamicallyallocated depending upon the information traffic flow in a given networksegment.

[0045]FIG. 3 shows a more general arrangement of the system 10, whichincludes a plurality of transmitter 12 _(n) optically connected via theswitch 22 to a plurality of receiver 14 _(m). Analogous to FIG. 2, eachtransmitter 12 _(n) transmits an optical signal 24 _(n) which includesone or more wavelengths 18 _(nj) through a waveguide 26 _(n) to an inputport 28 _(n) of the switch 22. It will be appreciated that eachtransmitter may include one or more sources to transmit and one or morewavelength signals. Likewise, each receiver may include one or moredetectors for receiving the signals.

[0046] An optical distributor 34 _(n), such as a demultiplexer 36 and/ora splitter 38, is provided in the input port 28 _(n) to distribute thesignal 24 _(n) to the waveband selectors 30 _(n,m). An optical combiner40 _(m), such as a wavelength division multiplexer 42 or a coupler 44,is generally included to combine the wavelengths 18 _(m,k) in waveband16 _(m,l) emerging from the waveband selectors 30 _(n,m) and provide amodified signal 24′_(m). The modified signal 24′_(m) exits the switchthrough the output port 32 _(m) and passes along waveguide 26 to thereceiver 14 _(m).

[0047] For convenience and clarity, FIG. 3 shows only a wavebandselector 30 connecting input port 28 _(l) to output port 32 _(l).However, it should be understood that the switch 22 will generallyinclude at least one waveband selector 30 between each input port 28 andeach output port 32. It is also noted that in some networks it is notnecessary that corresponding input and output ports, e.g. 28 _(l) and 32_(l), be connected to loop a signal back to its point of transmission.In addition, reference numeral subscripts are generally not used in theremainder of the description to simplify the nomenclature.

[0048] Transmitters 12 used in the system 10 can include one or moreoptical emitters and sources that provide continuous wave and/or pulsedbeams, such as one or more modulated lasers as is known in the art. Thetransmitter 12 may also include narrow band incoherent sources such asdescribed in U.S. Pat. Nos. 5,191,586 and 5,268,910 issued to Huber orother optical sources for producing optical signals. Information can bedirectly or indirectly, e.g., externally, modulated, or alternativelyupconverted, onto an optical wavelength, and the information itself maybe a time division multiplexed signal.

[0049] The transmitter 12 can also be used to provide multipleinformation carrying wavelengths using techniques such as described inU.S. Pat. No. 5,400,166. Multiple information carrying wavelengths canbe placed on a single carrier from the transmitter 12 using techniques,such as subcarrier modulation (SCM). SCM techniques are described inU.S. Pat. Nos. 5,101,450, 5,134,509, and 5,301,058 issued to Olshansky,4,989,200 issued to Olshansky et al., 5,432,632 issued to Watanabe and5,596,436 issued to Sargis et al.

[0050] The transmitters 12 may be coupled to an external electricalnetwork or part of an optical-electrical-optical (O/E/O) signalregenerator within an optical network. One skilled in the art willappreciate that the selection of the transmitter 12 and the number ofinformation carrying wavelengths will depend upon the desiredinformation transfer rates for a particular transmitter/receiver systemat the respective nodes. While the present invention provides theability to substantially upgrade the transfer rate for the node, it doesnot require that older, slower nodes be upgraded upon implementation ofthe present invention.

[0051] Consistent with the discussion regarding the transmitter 12, thereceiver 14 and transmission fiber 26 does not have to be upgraded to becompatible with the present invention. In the present invention, thecapabilities of the receiving system can be taken in account whenestablishing wavebands to be transmitted to a particular receiver 14.

[0052] As shown in FIG. 4, the receiver 14 will generally be used toseparate the individual information carrying wavelengths 18 _(i,k) ineach waveband 16 _(i) contained in the modified signal 24′ and convertthe information to one or more electrical signals. The receiver mayinclude a number of a wavelength filters, such as Bragg gratings ordemultiplexers, in combination with an optical to electrical converter(O/E), such as a photodiode, to provide for direct detection of theindividual wavelengths. The receiver 14 may also provide for indirectdetection of the individual wavelengths, such as by using coherentdetector arrangements.

[0053] Referring to FIG. 5, the system 10 may include other types ofintermediate processing nodes 20, such as add and/or drop devices. Theother intermediate processing nodes can be employed to selectivelymodify the wavebands in the signal 24′ and pass a further modifiedsignal 24″ to successive switches 22 and to the receivers 14. Thesubsequent switches 22 between other intermediate processing nodes 20and the receivers 14 can be used to further process the signal 24″ toproduce a further modified signal 24′″ which may include waveband subset16 _(il). The optical add and/or drop devices/ports can be embodied as a2×2 switch that can provide for 100% programmable add/drop capability orby employing directional devices, such as couplers and/or circulators,with or without waveband selectors 30 to provide varying degrees ofprogrammability, as will be further discussed.

[0054] The receiver 14 can also be used to further distribute the signal24′″ as a part of an O/E/O signal regenerator. One skilled in the artwill appreciate that in an O/E/O regenerator the optical wavelengthsreceived by the receiver 14 do not necessarily have to correspond to theoptical wavelengths at which the information is further transmitted.

[0055] Waveband selectors 30 generally include at least one filter,gate, and/or switch configured to pass and/or substantially prevent thepassage of at least one waveband 16 received from the inlet port 28 tothe outlet port 32. A signal is generally considered to be substantiallyprevented from passage, if the signal is sufficiently attenuated suchthat a remnant of the attenuated signal that passes through the wavebandselector does not destroy signals that have been selectively passedthrough the optical processing node 20. For example, a 40 dB attenuationof a signal will generally be sufficient to prevent cross-talkinterference between remnant signals and signals being selectivelypassed through the optical processing node 20.

[0056] In an embodiment shown in FIG. 6, the switch 22 includes awaveband demultiplexer 36 and an optical signal splitter 38 coupled viaa doped optical fiber 46 to the multiplexer 42 at the output port 32.When an optical signal is to be passed to the output port 32, the dopedfiber is supplied with energy from the switch pump 48 to overcome theabsorption of the doped fiber 46. The amount of energy supplied by thepump 48 can be controlled to selectively amplify or attenuate a signalbeing passed through the waveband selector 30. In the absence of opticalpump energy, the doped fiber 46 will absorb the optical signal, therebysubstantially preventing the passage of that portion of the signal tothe outlet port 32. In the embodiment of FIG. 6, the wavebands can beswitched to any number of output ports including one to one switchingand one to many broadcasting.

[0057] The dopant in the doped optical fiber 46 can be erbium or anyother dopant including other rare earth elements that can render thefiber transmissive in one state and substantially less transmissive inanother state. The selection of a dopant in the doped fiber will dependupon the information carrying wavelengths that are to be switched in thesystem. Also, mechanical, electro-optic, liquid crystal, semiconductor,and other types of switches along with gratings, filters and gates, canbe substituted for or used in combination with doped fiber 46 to achievedesired characteristics in the switch 22.

[0058] The waveband selector 30 may include reflective (≧50%reflectance) and/or transmissive (≦50% reflectance) selective elementsthat can be used to pass, either reflect or transmit, any of thewavebands 16 that comprise the signal 24. The waveband selector 30 mayemploy Mach-Zehnder filters, Fabry-Perot filters, and Bragg gratings toperform the waveband selection.

[0059] As shown in FIGS. 7 and 8, waveband selectors 130 and 230,respectively, can include a plurality of in-fiber reflective Bragggratings 50 (FIG. 6) and/or transmissive Bragg gratings 52 (FIG. 7) topass selected wavebands to the output ports 32. Each grating, 50 and 52,can be provided to pass selected wavebands to output ports 32.Alternatively, the waveband selector 30 may include a series of multipleBragg gratings that provide for piecewise coverage of the waveband. Inthe case of a multiple grating waveband selector, some separation of thewavelengths in the waveband will occur between gratings, but themultiple gratings are collectively operated to pass or substantiallyprevent the passage of the waveband. The multiple grating selector canbe tuned to individual idler gaps or telescoped to one or more commonidler gaps to decrease the idler gap bandwidth.

[0060] The number of gratings in FIGS. 7 and 8 is shown as being equalto the number of wavebands 16 being switched. However, the number ofselectors provided in the switch does not necessarily have to correspondto number of wavebands 16 currently in the system. For example, theconfigurations shown in FIGS. 6-12 may also be suitable for use inadd/drop multiplexers, as well as demultiplexers or multiplexers, inwhich any number of wavebands can be processed.

[0061] It may also be advantageous to provide sub-wavebands within thewavebands 16 of varying size that can be received, divided into thesub-wavebands, and the sub-wavebands can further transmitted to otherreceivers within the system. The waveband selectors 30 can also be usedto pass multiple wavebands to reduce the number of components in thesystem 10. In addition, the wavebands 16 can be selected to overlap toallow one or more wavelengths 18 to be transmitted in multiple wavebands16.

[0062] As shown in FIG. 7a, a waveband selector 130 can include a threeport circulator 54 used in conjunction with the plurality of reflectiveBragg gratings 50 using a configuration similar those discussed in U.S.Pat. Nos. 5,283,686 and 5,579,143 issued to Huber, and 5,608,825 issuedto Ip. In FIG. 7b, a waveband selector 230 employs transmissive gratings52 to transmit selected wavebands to the output ports 32 and reflect theremaining wavebands. An optical isolator 56 can be incorporated toprevent reflected wavebands from propagating back to the input ports 28.One skilled in the art will appreciate that directional couplers andother directional devices can be substituted for the optical circulatorswith appropriate circuit modifications.

[0063] The optical processing node 20 may include a wavelength converter58 to provide for switching one of more of the wavelengths in thetransmitted signal 24. In FIG. 7a, the wavelength converter 58 is shownbefore the waveband selector 30; however, the wavelength converter 58may also be positioned after the waveband selector 30 and operatedaccordingly.

[0064] Similarly in FIG. 8, a waveband selector 330 can be used with oneor more directional devices, such as a circulator or a coupler, witheither reflective or transmissive waveband gratings, 50 _(i) or 52 _(i),to select wavebands. It will be appreciated that the selector 330 can beemployed as an add and/or drop device/port, as well as a filter or in ademultiplexer or multiplexer in the system 10.

[0065] The optical distributor 34 associated with the input port 28 canbe embodied as an optical splitter to split the signal 24 and distributea portion of the entire signal 24 to each of the output ports 32. Asshown in FIG. 9a, the optical distributor 34 can be embodied as acirculator 54 to provide the entire signal to each waveband selector430. Wavelengths within waveband of the selector 230 are transmitted tothe output port 32, while the remaining wavelengths are reflected by thetransmissive gratings and circulated to successive ports.

[0066] Likewise, optical couplers can serve as the distributor 34 toprovide the entire signal to waveband selector 530 (FIG. 9b). Oneskilled in the art will appreciate that directional devices, such asmultiple three port circulators and/or coupler, can be cascaded invarious other configurations equivalent to those shown in FIGS. 9a&b.The gratings, 50 or 52, could be prepared having a reflectivity andtransmittance of less than 100%, to allow a portion of signal to betransmitted and reflected.

[0067] The fiber Bragg gratings 50 and 52 used in the switch 22 can bepermanently and/or transiently produced. Embodiments of the presentinvention incorporate fixed and/or tunable permanent Bragg gratings, 50and 52 as the waveband selectors 30. The permanent gratings used in thepresent invention can be prepared by conventional methods, such as byusing ultraviolet (UV) light to irradiate a GeO₂ doped fiber core. Suchmethods are discussed in U.S. Pat. Nos. 4,725,110 issued to Glenn etal., 5,218,655 and 5,636,304 issued to Mizrahi et al., which areincorporated herein by reference, and related patents.

[0068] The permanent gratings can be tuned to provide for reflectance ofa waveband in one mode and transmittance in another mode. Tuning of thegrating properties can be accomplished mechanically (stretching),thermally, or optically, such as discussed in U.S. Pat. Nos. 5,007,705,5,159,601, and 5,579,143, and by M. Janos et al., Electronics Letters,v32, n3, pp. 245-6, electronically, or in some other appropriate manner.

[0069] A limitation of tunable permanent gratings is that a portion ofthe wavelength band can not be used to transfer signals. The unusedportion of the wavelength band, called an “idler” gap, is necessary toprovide each permanent grating with a gap in the wavelength spectrum inwhich the grating will not affect a signal encountering the grating.

[0070] Transient reflective or transmissive gratings, 50 ^(T) and 52^(T), respectively, could also be used in the waveband selector 30.Transient gratings can be used to reduce or eliminate the need for idlergaps in the transmission wavelengths and provide increased flexibilityin the wavelength selectivity of the switch 22.

[0071] Transient gratings, either 50 ^(T) or 52 ^(T), can be formed in aportion of the fiber in which the refractive index of the fiber can betransiently varied to produce a grating. In an embodiment, the fiberportion is doped with Erbium, other rare earth elements, such as Yb andPr, and/or other dopants that can be used to vary the refractive indexof the fiber to produce a grating. In another embodiment, the transientgrating can be formed in a fiber section that contains a permanentgrating to provide a combined performance grating and/or to establish adefault grating in the absence of the transient grating.

[0072] As shown in FIGS. 10-12, transient gratings can be written byintroducing a grating writing beam either directly into the transmissionfiber or by coupling the writing beam into the transmission fiber. Oneor more transient grating writing lasers 60 _(i) are used to introduce atransient grating writing beam into the doped portion of the signalwaveguide 26. In a waveband selector 630 shown in FIG. 10, the writingbeam is split into two paths and introduced into the transmission fiber26 via ports 62. A plurality of narrow wavelength reflective gratings 64_(i) are positioned in one of the writing beam paths to control theposition of the standing wave in the waveguide 26 by introducing a timedelay on the wavelengths of the writing beam. Narrow wavelengthreflective or transmissive gratings, 64 _(i) or 66 _(i), can also beused to remove the writing beam from the transmission fiber 26.

[0073] As shown in FIG. 11, the writing beam can also be reflected backupon itself using spaced narrow wavelength reflective gratings 64 _(i),to form a standing wave and produce a transient gratings 50 ^(T) inwaveband selector 730. The grating writing lasers 60 _(i) can beoperated in conjunction with modulators 68 and pulsing switches 70 tocontrol the coherence of the writing lasers 60 _(i) and the resultingtransient gratings 50 ^(T) _(i). A waveband selector 830, shown in FIG.12, can also be configured with a reflector 72 in a coupled fiber toestablish a standing wave by reflecting the writing beam back uponitself to form the standing wave in a manner similar to that describedwith respect to FIG. 11.

[0074] Single wavelength continuous writing beam arrangements have beenused for signal identification and pattern recognition as discussed byWey et al., “Fiber Devices for Signal Processing”, 1997 Conference onLasers and Electro-Optics, Baltimore, Md. Also, U.S. Pat. No. 5,218,651issued to Faco et al., which is incorporated herein by reference,describes two beam methods for producing a transient Bragg grating in afiber.

[0075] In systems 10 of the present invention, the switch 22 can be usedto optically connect a transmitter and a receiver (FIG. 2) in a 1×1configuration or a plurality of nodes 100 in an n×m configuration (FIGS.13-14). In a 1×1 configuration, the switch 22 can be useful for droppingwavebands or for varying the waveband characteristics (gain trimming) ofthe signal.

[0076] The nodes 100 used in the system 10 may contain various systemcomponents including optical transmitters, receivers, and/or otherprocessing equipment, such as switches depending upon whether the nodeis an origination (transmitting signals) and/or a destination (receivingsignals) node, and whether it is a terminal node. The system 10 mayfurther include other optical transmission equipment, such as opticalamplifiers 74, and other optical processing nodes 20, such as opticaladd/drop multiplexers, between the switches and the nodes 100 as may beuseful in a given system.

[0077] The 4×4 switch arrangement shown in FIG. 13 is representative ofa north-south-east-west communication system. One skilled in the artwill appreciate that the nodes/switch arrangements can be varied toaccommodate various network configurations. For example, a 3×3arrangement is shown in FIG. 14. The arrangements in FIGS. 13 and 14show the cross connections of the switches 22, but do not show thewaveband selectors within the switches 22.

[0078] The flow of communication traffic between the nodes can takeplace using a variety of optical waveband hierarchies. In an embodiment,the optical wavebands are established and wavelengths assigned based onboth the signal origination node and the signal destination node toavoid the need for wavelength conversion in the optical network.

[0079] For example, the spectrum of wavelengths used with each receivercan be divided into wavebands and the destination wavebands assigned totransmitters. The assignment may be static or dynamically controlled atthe network management level so no overlap occurs in the wavebandsassigned to each transmitter from the various receivers. Dynamic controlof the waveband assignment provides flexibility in the wavelengthmanagement in the system 10 and can be performed at various points inthe system, such as at the client system, e.g., SONET, SDH, ATM, IPinterface with the optical network.

[0080] Waveband hierarchies in which the origination and destinationnodes are paired are particularly useful in communication systems inwhich a signal is being sent from the origin to one destination, such asin telephone communication systems. In addition, the present inventioncan also accommodate the necessary protection systems to providemultiple paths to the same destination by proper allocation of thewavelengths.

[0081] In a multiple destination system, such as a cable televisionsystem, it may be more appropriate for the wavebands to be determinedbased solely on the origination node of the signal. Waveband selectorscan be included in the switches 22 to pass signals corresponding to aparticular source to any number of destination nodes. The switch 22 canprovide further control over the distribution of signals by passingbroadcast signals to a distribution segment only upon a subscriber'srequest. The CATV provider, in response to a programming request, cancentrally control the switch to deliver the signal to the requester. Inthe absence of an express request by a subscriber the signal would notbe broadcast to the segment. The limited availability of the signal on asegment may discourage pirating of programming signals.

[0082] Switches 22 of the present invention can also be used for remoteswitching and routing of communication traffic in the event of a faultin the system. For example, in FIG. 13 if a signal were to travel fromnode A to node C, the typical path would be through the switch connectedbetween nodes A and C. However, if a fault occurs in the line from theswitch to node C, it may be desirable to route traffic from node Athrough node D to node C. Upon detection of the fault, the networkmanagement system could reconfigure the switches 22 in the system 10 toreroute the traffic or switch to a previously configured protectionroute.

[0083] As can be seen, the present invention provides for flexibility inoptical transmission systems. In addition, the present inventionprovides for increased transmission capacity without the commensurateincrease in complexity that was present in the prior art systems.

[0084]FIG. 15 is a block diagram of methods that can be used to formsystems 10 and SGRs 11 using spectral groups according to the presentinvention. Those methods allow for reducing the amount of regeneration,efficiently utilizing bandwidth, and providing a high degree ofreliability. Those methods may be performed individually or incombination with all or some of the other methods, or in conjunctionwith other methods, such as span engineering. Those methods includerouting connections 620, selecting regeneration sites 622, formingspectral groups 624, assigning spectral group numbers 626, and assigningchannels 628. Each method will be discussed hereinbelow. Advantages ofthe methods can be realized with all-optical systems as well asoptical/electrical systems, although most of the discussion will befocused on all-optical systems 10.

[0085] The method of routing connections 620 will be described withrespect to FIGS. 16-18. The method includes determining or specifyingthe topology of the system 10, including the endpoints of each link andthe link distance. The method also includes determining or specifying aset of demands, such as the data rate and the desired level ofprotection. The method also includes determining or specifying otherparameters, including span loss, fiber type, etc.

[0086] For all-optical systems 10, the method 620 includes shortest pathrouting as the starting point, as will be further explored hereinbelow,because costs generally increase with the number of regeneration sites.In contrast, the method includes minimum hop routing for systems whereregeneration occurs at every node, because minimizing the number of hopsminimizes the number of nodes that are crossed and, therefore, thenumber of regenerations. Advantages of the present invention may berealized with both shortest path routing and minimum hop routing, andcombinations thereof.

[0087] The routing method 620 may be performed for both unprotecteddemands and protected demands, and includes determining the protectionlevel on a per-demand basis or on a group or some other basis. Forunprotected demands, routing includes using a shortest path method. For1+1 protection, shortest dual-path routing can be used, where, ifpossible, the resulting primary and secondary paths are node and linkdisjoint. In general, the shorter of the two paths is taken to be theprimary path.

[0088] The routing method 620 includes forming dual paths that are notthe shortest path plus the second shortest path. As shown in thehypothetical network of FIGS. 16a and 16 b, the shortest path betweenOmaha and Los Angeles is shown by the thick lines. However, selectingthis path precludes a second path that is completely link and nodedisjoint. The optimal dual-path is shown in FIG. 22.

[0089] The routing method 620 includes a mode in which ‘shortest’ is theshortest distance. In this mode, routes are chosen such that theend-to-end path (or the combination of the primary and secondaryend-to-end paths) has minimum distance. In another mode, the methodincludes factors such as regeneration sites, minimizing regenerationsites, optical equipment penalties, and noise figures, so that routesare defined using factors such as the Optical Signal to Noise Ratio(OSNR), which is associated with the Noise Figure associated with eachlink. In that mode of operation, routing includes ‘minimum noise figure’routing.

[0090] Another factor that can be used is “optical penalty”. Each pieceof optical networking equipment (e.g., Router, OADM, etc) has an opticalpenalty associated with it. When calculating shortest path (whetherbased on distance or noise figure), the routing method 620 includesdetermining the optical penalties of routes. For example, the method cancalculate an effective distance for each link that is comprised of theactual distance of the link, plus half of the optical penalty at onelink endpoint, plus half of the optical penalty at the other endpoint.Using half of the penalty at each end allows the link direction to beignored when calculating shortest path. If a node is an intermediatenode on a path, the path will contain a link into and out of the node,thus, the whole optical penalty will be counted at that node. For endpoints half of the optical penalties can be counted, although whendetermining the shortest path between a set of endpoints, the penaltiesat the endpoints do not play a role since they will be the same for allpaths.

[0091] Furthermore, the method 620 includes distinguishing between thepenalty suffered by traffic that optically passes through a node 11 asopposed to traffic adding/dropping at the node 11, and assigningdifferent optical penalties for the different traffic patterns. Forexample, optically passing through an OADM will result in a noise figureand distance penalty. If signal channels are added/dropped at an OADM,the corresponding penalties may be different.

[0092] In addition to determining the shortest path, the method 620 alsoincludes selecting one or more possible alternative paths for eachconnection. The method 620 includes generating a list of possible paths.For example, the method 620 includes routing all connections along theirmost desirable path (without regard to link capacity), determining wherethe hot-spots are in the network (i.e., the spots in the network thatare likely to become congested), searching for alternative paths thatavoid these hot spots (while enforcing additional rules such asalternative paths should have no more than N extra regenerations—we havefound that N can often be set to 0 and still provide useful alternativepaths), and identifying one or more of the alternative paths. In somecases, an alternative path may have fewer regenerations than the“shortest” path (e.g., due to electronic termination sites beingpresent, or due to the actual lengths of the primary and secondary), inwhich case the method includes selecting the alternative path as thepreferred path.

[0093] After generating the shortest path and any alternative paths foreach connection, routing of the demands is performed. For eachconnection, the method includes choosing a path such that the network isleft in the least-loaded state (i.e., the Least-Loaded algorithm). Inother words, the method identifies the “most loaded link” in the path,and can chose the alternate path in which the most load link carries thelightest load.

[0094] In general, it has been found that the relatively sparselyinterconnected systems 10 do not result in a large number of alternativepaths. Thus, alternative path routing produces only small gains in thesenetworks. It has also been found that, for purposes of efficientlypacking spectral groups, routing connections over the same path, ratherthan splitting them up over several paths, produces somewhat betterresults.

[0095] As systems 10 become more heavily loaded, the shortest path, orshortest dual-path, for a particular connection may not have enough freecapacity to support the connection. Rather than immediately add a secondfiber to the congested links, the method includes searching foralternative paths when such new demands are made. Even if thealternative path requires more regeneration, it may be less expensivethan populating a second path 26 with amplifiers and upgrading the nodalequipment to support the additional path 26.

[0096] In some systems 10 it is desirable to provide 1+1 protectionpaths, but the presence of spurs or other degenerate topologies in thetopology prevents completely node- and link-disjoint primary andsecondary paths. The routing method includes identifying common nodesand/or common links which prevent complete disjoinder and, ifappropriate, authorizing the use of common nodes or links for protectionpaths. In that case, the method includes finding the shortest dual path,such that the number of common nodes/links is minimized. If in therouting process, the shortest such path does not have free capacity,alternative paths can be considered, with or without limitations on thenumber of common nodes/links. FIG. 17 illustrates an example of 1+1protection over a common link and nodes (i.e., nodes D and E and thelink therebetween). In that example, the degenerate case is not a spur,although a system 10 having a spur is analogous.

[0097] The method of the present invention can allow for common nodesand common links to be authorized. As another example, in the network ofFIG. 18, if the method allows for common nodes but not common links, aprimary and secondary path can be found from A to E using common node B(i.e., paths ABCE and ABDE), but not from A to F, which would requirethe authorization of link EF to be in common.

[0098] The method for selecting regeneration sites 622 will be describedwith respect to FIG. 19. The method 622 is particularly relevant inall-optical systems 10. The method 622 can be used in conjunction withone or more other methods, such as to form all-optical systems 10, or itmay be used alone to plan regeneration sites. The method 622 has severalfeatures, one or more of which may be used.

[0099] The need to regenerate channels, or optical reach, in a spectralgroup depends on the maximum distance that the channels can travelwithout loss of data. The optical reach can vary from channel to channeldepending on such factors as fiber type, amplifier span lengths, opticalequipment penalties, etc. There is often flexibility as to whereregeneration can occur, as illustrated in the example of FIG. 19. Assumethat the connection from A to D needs to be regenerated once, and thatregeneration can occur at either node B or C. In general, spectral grouppacking is more efficient when regenerations occur at the same site.Thus, the method can be made to choose certain sites for regeneration.For example, node B may be consistently chosen over node C as theregeneration site. This is not to say that the site becomes a dedicatedregeneration site. Connections not requiring regeneration will stilloptically bypass the site.

[0100] Also, the method can specify some sites as ‘not preferred forregeneration’ (e.g., an office with limited space). At such sites,regeneration will only occur if absolutely necessary. Thus, in theexample of FIG. 19, if B is a non-preferred regeneration site, node Cwould be chosen instead.

[0101] The method can determine whether the system 10 should include anoptical/electrical hybrid portion including dedicated regenerationsites. For example, if a link has a distance of 6,000 km, and if thesystem reach is 4,000 km, a dedicated regeneration site could be addedalong this link in an appropriate location. The site could be, forexample, ‘back-to-back’ transmitters and receivers.

[0102] The regeneration method can also select regeneration sites tovary the amount of equipment required in the system 10. Selectiveregeneration of the signal channels in one or more spectral group can beperformed at a node 11 to avoid the need to deploy a regeneration nodewhere such a node would not otherwise be needed.

[0103] Regeneration site selection can also take into account otherfactors, such as the distance, accumulated noise figure, OSNR, etc. Forexample, if the method is in the ‘route on distance’ mode, regenerationis based on the length of the links and the ‘distance penalties’ of theequipment.

[0104] Another factor is the maximum number of optical nodes 11 that canbe transited before transients and/or accumulated crosstalk becomes aproblem. If a threshold is crossed, the signal is regenerated,independent of the length of the path.

[0105] The method of forming spectral groups 624 will be described withrespect to FIGS. 20-23. As with the other methods, the method of formingspectral groups 624 may be used in conjunction with one or more othermethods, or it may be utilized by itself. If the method is used to forma system 10 using spectral groups, it can be advantageously utilizedafter the methods of routing connections 620 and selecting regenerationsites 622.

[0106] Spectral groups typically are designed to have a maximum numberof channels, and can be populated with any number of channels from zeroto the maximum.

[0107] The method of forming spectral groups 624 includes determiningwhether the segment of the connection that falls between tworegeneration points (or between an endpoint and a regeneration point) isa subconnection (if a connection has no regenerations, then thesubconnection is equivalent to the connection). In general, spectralgroups are formed by grouping subconnections, not end-to-endconnections.

[0108] Initially, all subconnections that have the same two endpointsare bundled together. It is usually desirable to minimize the number ofbundles formed. An important feature of the method is grouping thechanmels in such a way as to form spectral groups with a high fill rate.However, there are situations where a bundle must be split into multiplesmaller bundles (e.g., moving some subconnections to a new bundle) Forexample, at SGR nodes it is advantageous to bundle togethersubconnections that will be transmitted and received at the samelocation. The subconnection bundle can be split such that allsubconnections in the resulting smaller bundles have the sametransmitter and receiver location. Also, if a bundle contains moresubconnections than the maximum number of channels in the spectralgroup, it needs to be split such that the resulting smaller bundlescontain no more than this maximum.

[0109] After the bundles are formed they are referred to as spectralgroups (SGs). The method includes determining how many spectral groupshave been formed on each link. The maximum number of SGs is a feature ofthe system 10 and the nodes 11 which is determined or provided earlierin the method.

[0110] If the maximum number of SGs is exceeded on a particular link,then the spectral grouping method includes combining or joining two ormore SGs together to form a single SG. Three processes will bediscussed: subsetting, merging, and branching. Those processes can makeuse of consolidation and/or broadcast features in various opticalprocessing devices to reduce the number of spectral groups used in theoptical link. However, such reduction typically results in some unusablebandwidth, as will be further described. One or more of the combiningmethods can be used in allocating connections to spectral groups andchannels.

[0111] The steps of combining SGs include checking that the constituentsubconnections can be combined into the same SG. For example, the totalnumber of subconnections assigned to channels in the resulting SG cannotbe greater than the maximum number of channels allowed. Other channelallocation rules, can also be included in the combining steps.Furthermore, the combining steps can include determining whether it ispossible to combine certain SGs because of the need to keep thesubconnections corresponding to certain primary and secondaryconnections in separate SGs.

[0112] The first SG combining method is subsetting. Consider the exampleshown in FIG. 20, which illustrates a SG that runs from Node 1 to Node 4that contains two subconnections, a second SG that runs from Node 1 toNode 3 that contains one subconnection, and a third SG that runs fromNode 2 to Node 4 that contains one subconnection. All three SGs can becombined to form a single SG that extends from Node 1 to Node 4, withthe resulting SG containing four channels, each of which is assigned toa subconnection.

[0113] The example of FIG. 20 illustrates the broadcast, or ‘drop andcontinue’, feature of the system. Individual wavelengths can be droppedfrom the SG at any node. It is not necessary that all wavelengths in theSG have the same source and destination nodes. In FIG. 20, for example,the SG drops channel 3 at Node 3, but the SG continues on to Node 4where the spectral group is terminated.

[0114] It is noted that in the FIG. 20 example, channel 3 is unavailablefor use on the link between Nodes 3 and 4, and channel 5 cannot be usedbetween Nodes 1 and 2.

[0115] The subsetting method can consider efficiency when performing thesubsetting operations. For example, a spectral group that extends over Mlinks has a capacity of M*N channel slots, where N is the maximum numberof channels per spectral group. The efficiency, or fill-rate, isdetermined by dividing the actual number of channel slots used by themaximum capacity. In the example shown in FIG. 21, the total capacity is3*8=24 channel slots. Of these, 18 are used, yielding a fill-rate of75%. Two factors contribute to the fill-rate: the amount of unusablebandwidth (for example, Channel 5 from Node 3 to Node 4), and the amountof unused bandwidth (for example, in the figure, there are only sevensubconnections in the SG). In lightly loaded networks fill-rate can benormalized based on the number of channels used in the SG.

[0116] Accordingly, the subset method can first join togethersubconnections to increase the fill rate where the resulting fill rateexceeds a certain threshold, or efficiency. If that is still notsufficient to produce a feasible solution, the method can reiterate,with the efficiency threshold lowered.

[0117] The merging method operates to merge channels in two SGs fromdifferent links that are the same or include at least one commonchannel. In the example of FIG. 22, one SG extends from Node 1 to Node4, and a second SG extends from Node 2 to Node 5, with both SGscontaining two subconnections. The two SGs can be merged into a singleSG that extends from Node 1 to Node 5. The method can check that thepath from Node 1 to Node 5 does not require regeneration due to theaccumulation of noise from the merged spectral groups. The method canalso check that no optical rings have been formed when merging the SGs.

[0118] In the example of FIG. 22, channels 1 and 2 cannot be used on thelink between Nodes 4 and 5, and channels 3 and 4 cannot be used on thelink between Nodes 1 and 2. As with subsetting, the percentage ofoverlapping links can be taken into account when the merge method isperformed.

[0119] The branching method can also be used to combine spectral groups.While subsetting and merging take advantage of the ability to drop oneor more channels at a node and continue the SG to further nodes, thebranching method takes advantage of broadcasting over multiple links ata node (such as a node equipped with an SGR). Consider the example ofFIG. 23. One SG runs from Node 1 to Node 5, another from Node 1 to Node6. The two SGs can be combined to form a single SG. In the West to Eastdirection, the optical router at Node 4 broadcasts the SG onto the linkbetween Nodes 4 and 5 and the link between Nodes 4 and 6. In the reversedirection, the optical router consolidates the signals from these twolinks. Note that it would also be possible to have a third subconnectionin this SG that extends from Node 1 to Node 4, whereby the opticalrouter would be performing a 3-way broadcast and consolidation.

[0120] In the example shown in FIG. 23, channel 1 is not utilized on thelink between Nodes 4 and 6, and Channel 3 is not utilized on the linkbetween Nodes 4 and 5. As with the other combining operations, thepercentage of overlapping links between SGs is considered.

[0121] The branching method tends to be more useful in a highlyinterconnected system 10 than in a sparsely interconnected system 10. Aswith merging, the noise accumulates with each link. Thus, in the exampleof FIG. 23, if the branching method calculates the OSNR, it should do soover all five links to determine if the resulting SG is within theregeneration bounds.

[0122] The method of forming spectral groups 624 will be described withrespect to FIGS. 20-23. The regeneration method may be usedindependently or it may be used in conjunction with one or more othermethods. For example, it may be performed prior to the routing step forforming spectral groups. Alternatively, the regeneration method may beused alone to plan regeneration sites. The regeneration method hasseveral features, one or more of which may be used.

[0123] The method of forming spectral groups 624 can be used to fit newsubconnections into existing SGs when new demands are added to anexisting network. For example, the method can fit subconnections intoSGs that have the same source and destination, analogous to the initialsteps of the spectral grouping method. For example, the first steps canbe fitting the new subconnections into spectral groups including thesame subconnections. If the number of resulting SGs (existing plus newones) is too high, the combining method goes into the combination mode,where it performs subsetting, merging, and branching, as describedabove. The method can initially move new subconnections into existingSGs as opposed to combining new SGs. The percentage of overlapping linksis considered when the new subconnections are combined with existing ornew SGs.

[0124] The method of forming spectral groups 624 can extend an existingSG. For example, an existing SG may extend between Nodes 1 and 2. A newsubconnection that extends from Node 1 through Node 2 to Node 3 may becombined with this existing SG. Thus, the existing SG would be extendedto include the link from Node 2 to Node 3. The switching processinvolved in extending the SG should not affect existing in-servicesubconnections.

[0125] The method of assigning spectral group numbers 626 will bediscussed with respect to FIG. 24. This method 626 can be used alone orin combination with other methods. For example, it can be used after thespectral group formation method 624. The spectral group assignmentmethod assigns each spectral group a number from 1 to the maximum numberof SGs. The SG numbers can be assigned such that two SGs are notassigned the same SG on a given fiber on a given link, or to anytransmitter/receiver. In FIG. 24, four SGs are shown. The two SGs thatride on the link between Nodes 1 and 2 cannot be assigned the samenumber (assuming there is just one fiber pair per link); similarly, thetwo SGs that drop at the same transmitter/receiver of Node 3 cannot beassigned the same SG even though they have no links in common.

[0126] In addition to selecting an SG number, the method can select afiber pair for each link of the SG (this is relevant for links withmultiple fiber pairs). The method can also make other selections, suchas selecting a transmitter/receiver or other device in situations whensuch selections are available.

[0127] The method of assigning SG numbers includes ordering the SGsbased on the difficulty of finding a satisfactory assignment. The numberof SGs that have been formed on each link and on each optical router ONGis tallied. Each SG is assigned a weighting based on its most heavilyutilized link or nodes. Thus, SGs on links or nodes 11 with the most SGswill be assigned first so that there is the most flexibility inassigning the SG number (i.e., as SGs are assigned numbers, there isless flexibility in choosing numbers for the remaining SGs since theyhave to avoid conflict with those already assigned). For SGs nodes onlightly loaded links or nodes, their assignment order can be based onthe number of hops comprising the SG. The greater the number of hops,the more difficult it is to find a free SG number on each one of thehops.

[0128] The method can use the Most Used scheme, where an SG number isconsidered for assignment based on the number of times it has beenassigned already. The rationale for this strategy is that the more timesa particular SG number is used, the harder it is to assign it to anotherSG, because the chance of conflict is higher. Thus, if it can beassigned without conflict, it is chosen. If the Most Used scheme doesnot successfully find an SG assignment that satisfies all SGs, a LeastLoaded scheme can be used. The least loaded links and nodes are chosenat each step if possible. In most scenarios there is only one fiber pairper link so that this scheme essentially operates as a First Fit schemethat takes into account loading at the node.

[0129] The method of assigning spectral group numbers can also considerfactors such as certain SG numbers have restricted optical reach oncertain fiber types. This can be taken into account when SG numbers areassigned. When the method utilizes that factor, it includes the step ofdetermining the fiber types comprising each SG and may include furthersteps to determine whether the fiber types vary on a span-by-span basis.Such assignment schemes include considering the SG numbers with thesmallest reach that are still greater than the length of the SG.

[0130] Another consideration when assigning SG numbers is that onlightly loaded systems, it is beneficial to have active wavelengths thatspan the full spectrum, so that the amplifier gain flattening algorithmswork properly. Thus, initially, SG numbers can be assigned so that sucha spread is achieved, if possible (e.g., SGs may be assigned in theorder 11, 5, 17, not simply 1, 2, 3).

[0131] Wavelength inventory can also be considered when assigningspectral group numbers. For example, customers may have certain Tx/Rxwavelengths in inventory that they wish to use. The SGs chosen based onthe methods described above may not match up with these wavelengths. Thespectral group method, however, can utilize the unused Tx/Rx modulesthat are located at each node and any Tx/Rx modules that are located ina central inventory location. The method can chose to match up with theTx/Rx in inventory as opposed to the SG numbers that would otherwise bechosen. Preference can be given to using the Tx/Rx that are located at aparticular node rather than the ones located in a central ‘bin.’ Thismode of operation may lead to a small decrease in the utilization of thenetwork because the SG assignment process may be less than optimal.

[0132] In general, the methods can be operated to avoid single points offailure when implementing protections schemes. For example, avoidingcommon nodes and links, avoiding common components within a node or, ifthat is not an option, avoiding common cards within a common component.

[0133] Even if the number of SGs formed on each link and ONG is lessthan or equal to the maximum allowable number, there is no guaranteethat the above SG assignment processes will find a feasible solution.There may be too many constraints such that a solution cannot be found.In this case, the SG bundling process can reiterate with a lowerefficiency threshold for combining two SGs into a single SG.

[0134] If further iterations of the methods still does not produce afeasible solution, then the method can return to the routing step. Forexample, the method can determines which link has the most SGs, andreduces the maximum number of paths that can be routed on that link. Thewhole process of routing, regenerating, SG bundling, and SG assignmentcan be performed again. The process can continue to iterate until asolution can be found.

[0135] The method of assigning channels will be described with respectto FIG. 20 and can be performed individually or in combination withother methods. If the channel assignment method is performed with othermethods to form spectral groups, it can be performed after the SGs havebeen assigned numbers.

[0136] The methods can operate so that SG and channel assignment isperformed on a subconnection basis, so that there is no attempt toassign the same SG number to all subconnections that comprise aconnection. Thus, whenever regeneration occurs (which partitions aconnection into subconnections), wavelength conversion typically occurs.Sparse wavelength conversion can be very effective in yielding highutilization in a system 10.

[0137]FIGS. 25a-25 d are flow charts illustrating one embodiment of themethod of forming spectral groups 624 described above. In FIG. 25a themethod begins by routing a connection between an origination node and adestination node. Routing the connection can be subjected to certaincriteria. For example, if no protection paths are required, routing aconnection may include finding the “shortest” route between theorigination node 11 and the destination node. The “shortest” path is notnecessarily the path with the shortest fiber length. For example,determining the shortest path may include considering factors such asfiber type, fiber condition, equipment condition, link distances, signalregeneration, signal degradation, amplifier types, and otherconsiderations and optical penalties of a particular path. If aprotection path is required, then two paths must be found. For example,in a 1+1 protection scheme, the two paths are node disjoint (except atthe origination and destination nodes) and thus link disjoint, which mayresult in the shortest protection path being longer than the shortestoverall path. Nonetheless, the shortest path that satisfies theprotection criteria and the second shortest path that satisfies thatcriteria are typically the primary and secondary paths, respectively. Atthe origination and destination nodes 11, which are shared even in 1+1protection schemes, criteria may include ensuring that the primary andsecondary paths are assigned to different transmitter/receiver equipmentand routers so as to avoid single points of failure in the originationand destination nodes 11.

[0138] Once the connections are made, they can be partitioned intosmaller sub-connections or sub-networks, such as various optical linksinterconnecting nodes 11 in the system 10, which is often desirable inlarge or long-haul systems 10. Thereafter, the connections and/orsub-connections that share the same paths are identified and bundledtogether to form spectral groups and sub-networks. One or more spectralgroups are assigned to those bundled connections and/or sub-connectionswhich share the same paths. The reason more than one spectral group maybe assigned is that the number of channels may exceed the capacity of asingle spectral group. If the number of assigned spectral groups is lessthan a maximum number allowed number of spectral groups, then individualsignal channels within the assigned spectral group can be assigned toeach information channel to provision the system 10.

[0139] In some networks, however, the number of information channels maybe sufficiently large that too many spectral groups are initiallyformed. As a result, the spectral groups and sub-networks must befurther refined to reduce the number of spectral groups. Such refiningcan be done using the aforementioned concepts of subsetting, mergingand/or branching, using various allocation criteria, as will bedisclosed in more detail below with respect to FIGS. 25b-25 d. Onecriteria that may be used in the subsetting, merging, and branchingoperations is a minimum acceptable efficiency of the refined spectralgroups. In that way, the subsetting, merging, and branching operationswill be forced to meet the minimum efficiency standards when refiningthe spectral groups. An inefficient system 10 can cause difficulties inthe future when additional modifications are made to the network, suchas adding capacity or changing traffic patterns.

[0140]FIG. 25b shows one embodiment of a subsetting operation accordingto the present invention. In that operation, each spectral group isconsidered with respect to every other spectral group, to determinedwhether spectral groups can be combined. An example of a three part testfor combining spectral groups is: (1) is the sub-network of one spectralgroup contained within a sub-network of another spectral group, and (2)is the efficiency of the combined spectral groups greater than or equalto the efficiency level set for the system 10, and (3) is the totalnumber of channels in the combined spectral group less than or equal tothe capacity of the spectral group. If the test for combining thespectral groups is satisfied, the spectral groups are combined, and ifthe test is not satisfied, the spectral groups are not combined. Themethod can be iterated until all subsetting operations meeting the testcriteria have been performed, or until the number of spectral groups iswithin the acceptable range.

[0141]FIG. 25c shows a method for performing a merging operation. Thatmethod is similar to the subsetting operation of FIG. 25b , except thatthe test for merging includes determining whether two sub-networks havean overlapping portion at opposite ends of the sub-networks. In theillustrated method, the test for merging also includes determiningwhether the merged sub-networks satisfy the efficiency criteria andwhether the merged sub-networks will exceed the capacity of a singlespectral group.

[0142]FIG. 25d shows a method for performing a branching operation. Thatmethod is similar to the subsetting and merging operations of FIGS. 25band 25 c, except that the test for branching includes determiningwhether the two sub-networks have an overlapping portion that is in themiddle of at least one of the sub-networks. In the illustrated method,the test for branching also includes determining whether the branchedsub-networks satisfy the efficiency criteria and whether the branchedsub-networks will exceed the capacity of a single spectral group.

[0143] After completing the subsetting, merging, and branchingoperations, the method can return to FIG. 25a to determine whether thereare still too many spectral groups. If the answer is no, the method offorming the spectral groups may terminate. If there are still too manyspectral groups, the method can lower the efficiency criteria, or changesome other criteria, and return to the subsetting, merging, andbranching operations to further refine the spectral groups andsub-networks. When returning to the subsetting, merging, and branchingoperations, the method may return the spectral groups to their originalform (as produced by the bundling operation of FIG. 25a) and re-performthe operations with the lower efficiency criteria. Alternatively, thesubsetting, merging, and branching operations may be performed on thespectral groups as they exist after the last subsetting, merging, andbranching operations. In the later case, however, the operations maybecome more complex, such as when calculating the efficiency of spectralgroups including two or more channels with different paths. Furthermore,by not returning the spectral groups to their original form, theoperations will lose the opportunity to make certain combinations whichwere prohibited by the efficiency criteria in a previous iteration.

[0144] The methods discussed with respect to FIGS. 25a-25 d areillustrative of the present invention. Variations on those methods, suchas searching and comparing only some of the spectral groups for thevarious subsetting, merging, and branching operations, as opposed tocomparing all subsets to all other subsets, can of course be utilized inaccording with the present invention. Furthermore, although the variousoperations are disclosed as being used together, one or more of theoperations may be used separate from the rest. Also, the order of thesteps and operations can be modified and still realize the benefits ofthe present invention.

[0145] In the present invention, systems 10 can be configured to operateas an all-optical or optical/electrical hybrid network in which thenodes in the network are assigned various nodal tiers depending upon therole of the node in the network. For example, a two nodal tier networkcan be constructed with major and minor nodes. The number of tiers usedin the network can vary depending upon how the artisan chooses toimplement the architecture. Each nodal tier in the network will beassigned various attributes to meet network design objectives.

[0146]FIG. 26 shows a network map of the system 10 including 60 nodes.In an embodiment, the network can employ two nodal tiers consisting of16 major nodes and 44 minor nodes. Major nodes can be assigned to nodesthat have significant traffic capacity requirements in terms of being asource, destination, or pass-through point in the network. Also, thesenodes could play a critical function in the network, such as being atthe intersection, or junction point, of a network where multiple pathsthrough the network intersect or connections are made to other networks.In addition, the major nodes can serve as aggregation hubs for trafficoriginating from or destined for minor nodes and/or from other networks.As such, the major nodes can include interfacial switching devicesand/or integrated switching devices that allow for the establishment ofoptical transmission paths across multiple fibers in the network orconnections are made to other networks.

[0147] Conversely, the minor nodal tier can include nodes that do nothave a significant traffic capacity requirement. The trafficconnectivity requirement of minor nodes may extend to or beyond adjacentmajor nodes. Traffic originating from a minor node can be extracted fromthe network at the adjacent major node, irrespective of whether thetraffic is destined for nodes beyond the adjacent major node. At theadjacent major nodes, the traffic can be aggregated with other trafficfrom other minor nodes or external network for transit throughout orbeyond the network. Likewise, traffic destined a minor node can betransmitted to an adjacent major node, where it will be separated fromthe other traffic and forwarded to the minor node.

[0148] The number of wavelengths assigned to nodes will depend upon thetraffic capacity of the nodes and the type of node. For example, majornodes can have one or more dedicated wavelengths assigned to aconnection to other major nodes in the network. The connectivityrequirements of major nodes can be defined based on the physical networkconnectivity and the traffic connectivity through the network. Thewavelengths assigned between two major nodes can traverse the same ordifferent paths through the network. For wavelengths traveling the samepaths, the wavelengths can be grouped into a common waveband, orspectral group, which can be handled by a single optical switchingdevice. The wavelengths dedicated between major nodes can be uniquewavelengths throughout the network and/or wavelengths that are unique toa sub-network of the network. For example, the same wavelength can beused to connect New York and Boston, as well Los Angeles and SanFrancisco, because these connections generally cover diverse pathsthrough the network.

[0149] One or more wavelengths can be assigned to minor nodes in thenetwork. Traffic can be added and dropped at the minor nodes usingvarious wavelength reuse and non-reuse add/drop devices. For example,wavelengths assigned to a minor node can provide connectivity toadjacent major nodes in the network. In various embodiments, thewavelengths from minor nodes are terminated at the adjacent major nodesand the traffic aggregated for further transmission in the network ordistribution to other networks. In this manner, the number ofwavelengths assigned to a minor node tier can be kept to a minimum.

[0150] As shown in FIG. 27, wavelengths assigned to minor nodes can bereclaimed at adjacent major nodes. In addition, one adjacent major nodecan provide a working path for the minor node traffic, while anotheradjacent node can provide a protection path using the same wavelength,if there is a failure of the working path.

[0151] The multiple tiered nodal architecture provides a basis for anall-optical express network that can also handle traffic from minornodes in the network without excessive switching equipment, electricalregeneration, or the implementation of separate back haul networks forminor nodes. In various embodiments, major and minor nodes areimplemented in an all-optical network, which includes optical switchesand add/drop multiplexers integrated into the transmission system toprovide all-optical connectivity between the nodes. In addition,electrical regeneration, switching, grooming, and/or aggregation can beperformed by interfacial switching devices working in conjunction withthe optical switches and add/drop multiplexers and transmitters andreceivers in the network.

[0152] It will be appreciated that additional tiering beyond two tierscan be performed to accommodate various network objectives. For example,a major node tier can be subdivided into two major node tiers withdifferent hierarchical attributes, such as capacity and/or line rates.Similarly, multiple minor node tiers can be assigned; for example, awavelength reuse minor node tier and a no wavelength reuse minor nodetier. It will be appreciated that this network architecture combines theefficiencies of grooming and aggregation to reduce the number ofwavelengths with the economies of scale offered by transparent opticalequipment.

[0153] Those of ordinary skill in the art will appreciate that numerousmodifications and variations that can be made to specific aspects of thepresent invention without departing from the scope of the presentinvention. It is intended that the foregoing specification and thefollowing claims cover such modifications and variations.

What is claimed is:
 1. An optical system comprising: a first networktier including a plurality of major nodes optically interconnected by atleast one transmission path; a second network tier including a pluralityof minor nodes disposed along the transmission path and the minor nodesare connected to at least one of the major nodes, wherein the minor nodeis configured to transmit all traffic to an adjacent major node and themajor nodes are configured to transmit to and receive information fromother major nodes and minor nodes on transmission paths connected to themajor node.
 2. The optical system of claim 1, further comprising a thirdnetwork tier of major nodes optically interconnected by at least onetransmission path, wherein the first and third network tiers havedifferent hierarchical attributes.
 3. The optical system of claim 2,wherein the first and third network tiers have different capacities. 4.The optical system of claim 2, wherein the first and third network tiershave different line rates.
 5. The optical system of claim 1, furthercomprising a fourth network tier of minor nodes disposed along thetransmission paths, wherein the second network tier and fourth networktier have different hierarchical attributes.
 6. The optical system ofclaim 5, further comprising wavelengths transmitted along thetransmission paths between the minor and major nodes, wherein the secondnetwork tier reuses wavelengths and the fourth network tier does notreuse wavelengths.
 7. An optical system comprising: a plurality of majornodes optically interconnected by at least one transmission path; atleast one minor node disposed along the transmission path connected toat least one of the major nodes, wherein the minor node is configured totransmit all traffic to an adjacent major node and the major nodes areconfigured to transmit to and receive information from other major nodesand minor nodes on transmission paths connected to the major node. 8.The optical system of claim 7, wherein the major nodes aggregate trafficfrom minor nodes.
 9. The optical system of claim 7, wherein the adjacentmajor node extracts traffic destined for the minor node and forwards thetraffic to the minor node.
 10. The optical system of claim 7, furthercomprising wavelengths transmitted along transmission paths between themajor nodes, wherein traffic is transmitted between a first major nodeand a second major node along a wavelength.
 11. The optical system ofclaim 10, wherein traffic is transmitted between a first major node anda second major node along more than one wavelength.
 12. The opticalsystem of claim 11, wherein the wavelengths transmitted between thefirst and second major nodes use different transmission paths betweenthe first and second major nodes.
 13. The optical system of claim 1 1,wherein the wavelengths transmitted between the first and second majornodes use the same transmission between the first and second majornodes.
 14. The optical system of claim 13, wherein the wavelengths aregrouped into a spectral group.
 15. The optical system of claim 10,wherein traffic is transmitted along a wavelength between a first set oftwo major nodes along a first path and along the same wavelength betweena second set of two major nodes along a second path where the first andsecond paths cover diverse paths through the transmission path.
 16. Theoptical system of claim 7, wherein the minor node has a first and asecond adjacent major node and the first adjacent major node provides aworking path for the minor node and the second adjacent major nodeprovides a protection path for the minor node.
 17. A method foroptically communicating in an optical network having first and secondnetwork tiers including a plurality of major and minor nodes,respectively, interconnected by at least one optical communicationspath, comprising: receiving at a major node traffic from a minor node;aggregating traffic received at the major node from the minor node;transmitting traffic from the major node to another node in the network.18. The method of claim 17, wherein the major node and the minor nodeare adjacent to each other.
 19. The method of claim 17, whereinaggregating includes aggregating traffic into a plurality of signalchannels based on traffic destination.
 20. The method of claim 17,wherein receiving includes receiving at the major node traffic from aplurality of minor nodes.
 21. The method of claim 20, whereinaggregating includes aggregating traffic received at the major node froma plurality of minor nodes.
 22. The method of claim 17, whereintransmitting includes transmitting at least one signal channel toanother major node.
 23. The method of claim 17, wherein transmittingincludes transmitting at least one signal channel to a different minornode.
 24. The method of claim 17, further comprising: receiving trafficat a major node, wherein less than all of the traffic is destined for aminor node; separating at the major node the traffic destined for theminor node; transmitting to the minor node the traffic destined for theminor node.
 25. The method of claim 24, wherein receiving includesreceiving traffic from a major node.
 26. The method of claim 24, whereinreceiving includes receiving traffic from a minor node.
 27. The methodof claim 24 wherein: separating includes separating traffic destined fora plurality of minor nodes; transmitting includes transmitting trafficto the plurality of minor nodes.
 28. The method of claim 17, furthercomprising: providing a working path between a minor node and a firstadjacent major node; and providing a protection path between a minornode and a second adjacent major node.